Codex 0.129.0-alpha.9 breaks Xcode Coding Assistant with decode error

What version of the IDE extension are you using?

Xcode Coding Assistant Codex agent: broken on codex-cli 0.129.0-alpha.9; works on codex-cli 0.120.0

What subscription do you have?

ChatGPT Pro / ProLite Codex account (login status shows "Logged in using ChatGPT")

Which IDE are you using?

Xcode 26.5 (Coding Assistant / Agents), XcodeVersions/17F42

What platform is your computer?

Darwin 25.4.0 arm64 arm; macOS reported by Codex as 26.4.1; Apple Silicon

What issue are you seeing?

After updating the Codex agent used by Xcode Coding Assistant from codex-cli 0.120.0 to codex-cli 0.129.0-alpha.9, Codex chat inside Xcode fails immediately.

Xcode shows:

Your request couldn't be completed.

The Diagnostics popover says:

The data couldn’t be read because it isn’t in the correct format.

This does not appear to be an auth or rate-limit issue:

• Xcode-specific Codex home: ~/Library/Developer/Xcode/CodingAssistant/codex
• Agent symlink: ~/Library/Developer/Xcode/CodingAssistant/Agents/XcodeVersions/17F42/codex
• Broken symlink target: ~/Library/Developer/Xcode/CodingAssistant/Agents/codex/0.129.0-alpha.9
• CODEX_HOME=$HOME/Library/Developer/Xcode/CodingAssistant/codex .../codex login status reports Logged in using ChatGPT
• A direct app-server smoke test with the same Xcode CODEX_HOME and agent returns OK
• Account rate limits shown by the direct smoke test are not exhausted: primary 16%, secondary 3%, rateLimitReachedType: null

Rolling the Xcode agent symlink back to 0.120.0 and restarting Xcode immediately fixes the same chat prompt. Xcode can then stream a response, read Package.swift, and answer normally.

What steps can reproduce the bug?

1. In Xcode 26.5, install/use Codex as an agent under Xcode > Settings > Intelligence > Agents.
2. Sign in with ChatGPT successfully.
3. Update the Xcode Codex agent to codex-cli 0.129.0-alpha.9.
4. Start a new Xcode Coding Assistant chat with the Codex agent.
5. Ask a basic project question, for example: Can you give me a quick overview of this Xcode project and what the Package.swift file is doing?
6. Observe Xcode fails with Your request couldn't be completed. Diagnostics says The data couldn’t be read because it isn’t in the correct format.
7. Verify auth/backend manually:

CODEX_HOME=$HOME/Library/Developer/Xcode/CodingAssistant/codex \
  ~/Library/Developer/Xcode/CodingAssistant/Agents/XcodeVersions/17F42/codex/codex login status

CODEX_HOME=$HOME/Library/Developer/Xcode/CodingAssistant/codex \
  ~/Library/Developer/Xcode/CodingAssistant/Agents/XcodeVersions/17F42/codex/codex debug app-server send-message-v2 'Say OK and nothing else.'

1. Roll back the symlink:

ln -sfn "$HOME/Library/Developer/Xcode/CodingAssistant/Agents/codex/0.120.0" \
  "$HOME/Library/Developer/Xcode/CodingAssistant/Agents/XcodeVersions/17F42/codex"

1. Restart Xcode and submit the same prompt again.
2. The Codex chat now works and streams a normal answer.

What is the expected behavior?

Xcode Coding Assistant should be able to use the updated Codex agent without failing to decode the app-server/protocol response. A successful ChatGPT login and working direct app-server smoke test should result in Xcode chat producing a normal answer.

Additional information

The working/broken comparison is very tight:

• codex-cli 0.129.0-alpha.9: Xcode chat fails with the decode-format diagnostic.
• codex-cli 0.120.0: Xcode chat works immediately after relinking and restarting Xcode.

I also tried disabling apps and plugins in the Xcode-specific config.toml because startup logs showed plugin sync warnings/403 HTML responses. Disabling those did not fix the Xcode failure on 0.129.0-alpha.9, so the issue seems more likely to be an app-server/protocol compatibility regression between the alpha agent and Xcode's Coding Assistant bridge.